In the case of high voltage power cables, lead sheaths and in more modern times, plastic-coated metal foil, have been used to prevent moisture from entering power cables. It was thought that moisture is necessary for the growth of electrochemical trees because it was common in underground cables, but applicants found that electrochemical trees grow in cables that are installed under conditions where there is complete sealing against entrance of moisture into the cable.
The invention of this application is based on the discovery that even where no moisture can enter a buried cable, the moisture was already there before the cable was buried. Although the trees grow from the inside surface of the insulation, that surface is sealed in a power cable against moisture because it was extruded over a semi-conducting shield.
We conceived the idea that air in a cable, even though the air contained relatively low humidity, might have enough moisture to support tree growth, and might come into the cable through the ends of the cable before the cable is installed, or at the time of making splices; and that the way in which the humidity reaches the inside surfaces of the insulation is by permeation as vapor through the insulation shield. The reason that the trees do not start in the insulation shield is because that shield is semi-conducting and therefore does not have electrical stresses high enough to cause electrochemical trees.
Unlikely as this humidity conception seemed to be, we made a high-voltage power cable with soft flexible sealant in those spaces of a bare stranded conductor which are not filled with the plastic material of the conductor shield. This was never done before in high voltage power cables, and with this construction, whatever length of cable might be cut, the ends of the cable were sealed against the entrance of atmospheric air into the cable with whatever moisture that air might contain. The cable of this application was tested under the operating conditions of a high-voltage power cable and for a long enough time for a conventional power cable to show tree formation; and there were no trees.
Some special problems arise which would not be presented in a different kind of cable. For example, it is desirable to limit the sealant to the inner interstices of a stranded cable and to leave the helical valleys on the outside surface of the conductor substantially free of sealant so that the semi-conducting material of the conductor shield can enter into these valleys and provide a more intimate contact between the conductor and the conductor shield. The sealant must e compatible with the material of the conductor shield; and the sealant must be capable of withstanding the high temperatures encountered in high-voltage power transmission cables.
Temperatures at the conductor commonly rise to 130.degree. C., and the sealant, though soft and flexible, must be viscous enough at that temperature to prevent the sealant from running or dripping or forming any gas.
Since the sealant extends for the entire length of the stranded conductor, it serves to seal the ends of the cable from entrance of humidity into the cable regardless of where a cable is cut to a required length for a particular installation.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.